Particle therapy systems use an accelerator to generate a particle beam for treating afflictions, such as tumors. In operation, particles are accelerated in orbits inside a cavity in the presence of a magnetic field, and removed from the cavity through an extraction channel. The particles are part of a beam, which is applied to the patient for treatment. The magnetic field is generated by a magnet, which produces magnetic flux. Too much stray magnetic flux can adversely affect the operation of the accelerator and of other components of the particle therapy system. A return may therefore be used to route the stray magnetic flux. Ferromagnetic returns can be heavy, and add considerable weight to the accelerator. This can be problematic in some cases.